Evaluating the Structural Basis for Polar Altermagnet Candidate Ca${}_3$(Ru,Ti)${}_2$O${}_7$

Abstract

The interplay between polar and altermagnetic orders remains largely unexplored in the broader landscape of correlated electron systems. Ca${}_3$Ru${}_2$O${}_7$ has been proposed by density functional theory (DFT) as a polar altermagnet, reliant on the transformation of experimentally reported $Bb{2}_{1}m$ phase to a lower symmetry $Pn{2}_{1}a$ structure. Here, we perform a targeted search for the $Pn{2}_{1}a$ phase using synchrotron X-ray diffraction on single crystals of Ca${}_3$Ru${}_2$O${}_7$ and Ca${}_3$(Ru${}_{0.99}$Ti${}_{0.01}$)${}_2$O${}_7$. No diffraction signature of the $Pn{2}_{1}a$ structure is detected down to 20 K within experimental limits of $\sim$60-200 fm atomic displacements, significantly smaller than the DFT prediction of $\sim$1 pm. Combined with recent nonlinear transport measurements, our structural study suggests Ca${}_3$Ru${}_2$O${}_7$ as a unique system where strong electron correlations drive an electronic phase transition without any measurable lattice symmetry change. With Ti substitution exceeding $\sim$3%, a chemically tunable altermagnetic phase with $Bb{2}_{1}m$ structure emerges. The study highlights the importance of sub-picometer metrology towards de-convolving structural versus electronic origins of altermagnets.